The invention relates to the field of MOSFET fabrication, and in particular to the formation of Ge channel MOSFETs grown on SiGe/Si virtual substrates.
Channel engineering in the silicon-germanium (SiGe) materials system can result in increased electron and hole mobilities over conventional bulk Si, leading to enhanced metal-oxide-semiconductor field-effect transistor (MOSFET) performance. In particular, increases in mobility (μ) realized through channel engineering lead to increases in MOSFET drain currents and ultimately to higher switching speeds. In addition, the low hole mobility of bulk Si (μhole˜0.5 μelectron) leads to increased p-MOSFET gate widths to compensate for their reduced drive currents. The increased chip area taken up by p-MOSFETs wastes valuable real estate, while the mismatch in n- and p-MOSFET areas further reduces logic speed through capacitive delays; both of which force circuit designers to avoid p-MOSFETs in logic circuits whenever possible. High mobility layers, while critical in n-MOSFETs, thus offer particularly important improvements for p-MOSFET design.
Compressively strained SiGe layers, deposited on bulk Si and capped with bulk Si to preserve the Si/SiO2 gate interface, lead to modest increases in hole mobility, though electron mobility is unchanged. Increased process complexity and degraded short channel effects further moderate gains in circuit performance through this channel architecture. Tensile strained Si layers grown on relaxed SiGe virtual substrates offer large gains in electron and hole mobility, but the ratio of electron to hole mobility remains unbalanced. Schottky-gated modulation-doped field-effect transistors (MODFETs) incorporating buried compressively strained Ge channels on relaxed Si1-xGex (x>0.6) virtual substrates provide high hole mobility, but their limited voltage swing, high standby power consumption, and process complexity preclude their use in digital or large-scale integrated circuits. The combination of buried compressively strained Si1-yGey channels and tensile strained Si surface channels on relaxed Si1-xGex virtual substrates (y>x), hereafter referred to as dual channel heterostructures, provide high hole mobility in a complementary MOSFET (CMOS)-compatible layer structure. Peak effective hole mobilities of 760 cm2/V-s have been reported for a dual channel heterostructure p-MOSFET with a strained Si0.17Ge0.83 channel on a relaxed Si0.48Ge0.52 virtual substrate.
Pure Ge has the highest hole mobility of all semiconductors, along with an electron mobility comparable to bulk Si. MOSFETs based on pure Ge channels thus offer large performance gains over bulk Si. Effective mobilities as high as 1000 cm2/V-s have been reported for n- and p-MOSFETs fabricated on bulk Ge and utilizing germanium oxynitride as a gate material. However, bulk Ge substrates are not an economical manufacturing technology for integrated circuits. Also, an effective hole mobility of 430 cm2/V-s has been attained for relaxed Ge deposited directly onto a (111) Si substrate with no buffers and utilizing a SiO2 gate. However, neither of these device structures provides the consistent control of defect density (imparted by SiGe virtual substrate technology) or well-developed gate interface (as, for example, in Si/SiO2) required for large-scale integrated applications.